1. Field of the Invention
The present invention relates to the field of network communications and in particular, relates to a system and method for improving the performance of a communication line such as an asymmetrical digital subscriber line (ADSL) by detecting the source of problems at a user end of the communication line.
2. Description of the Related Art
Various Digital Subscriber Loop (DSL) communication systems utilize the telephone local subscriber loops to carry high speed digital transmission. Examples of DSL services include asymmetric DSL (ADSL), high-rate DSL (HDSL), very high-rate DSL (VDSL) and others. The different types of DSL service are generally referred to as xDSL. The xDSL services share the telephone wires with traditional telephony, commonly referred to as plain old telephone service (POTS).
xDSL and POTS use different frequency bands for communication; POTS signals are restricted to frequencies below 4 kHz, while xDSL signals use frequencies greater than 4 kHz. The frequency band plans depend on the specific XDSL technology, e.g. ADSL-1 uses˜25-1100 kHz. At the end of the communication line, the xDSL and POTS channels are electrically seperated by using filters. These filters come in two main forms: one is the POTS splitter that splits (and combines) DSL and POTS signals, while the other is the inline filter that only allows POTS signals to pass through. FIG. 1 shows the configurations in which the splitter and inline filter is used. In this document, we do not distinguish between the two types of filter devices, and simply refer to them as microfilters, filters, DSL filters, or DSL microfilters.
FIG. 1 illustrates the installation configuration of the two commonly used DSL microfilters, typically installed at the end user location for the network related to the present invention. FIG. 1 shows an inline filter 101 that is often installed between an outside connection 103 and a local telephone 105. An inline filter usually comprises a low pass filter (bandwidth of about 4 kHz) for passing lower frequency POTS signals. FIG. 1 shows a 3-port splitter with a line port 114 for carrying both POTS as well as DSL signals. A POTS port 118 connects to the telephone and carries POTS, and port 116 that connects to the DSL modem carries DSL signals. A 3-port splitter generally combines a high-pass filter for passing DSL signals and a low-pass filter for passing POTS signals. The splitter can be installed by the service technician or self-installed, wheareas the inline filter is almost always self-installed.
The microfilter reduces the DSL power imparted on to the telephone electronics, protecting the xDSL channel from the harmonics and intermodulation products generated due to the non-linearities present in most telephones, referred to as non-linear echo in this document. Specifically, for ADSL, if upstream signals are allowed to reach the telephone device, they can be upconverted in frequency due to non-linearities in the telephone device and cause interference in the downstream channel. The microfilter is critical in reducing this effect. In this document, upstream refers to the communication from the customer premises equipment or CPE to the central office or CO, while downstream refers to communication from CO to CPE. The microfilter also protects the xDSL service from transients caused by the telephone going from on-hook to off hoof (and vice-versa), as well as associated impedance changes. Not using a microfilter can severely degrade the performance of the DSL channel.
Since microfilters are typically distributed to new DSL customers for self-installation, it is possible, even likely that a certain percentage of customers will neglect to install these filters. It is also possible that the microfilters are not connected in front of each and every telephone in the home. In short, there can be many reasons why microfilters do not get installed as required, putting the DSL channel at risk from the POTS service.
Therefore, for DSL service providers, it is important to diagnose this condition in order to correct it and ensure a high quality DSL channel. A prior method for detecting missing microfilters tends to measure the nonlinear echo on a line directly. Excessive levels of nonlinear echo can be an indication of that a DSL micro-filter is missing. However, this method is not currently engineered into existing installations; it is not a part of upcoming ADSL standards; and upgrading current installations to newer CPEs that can detect nonlinear echo can be costly.
Sending a repair truck to diagnose a missing micro-filter is extremely costly and inefficient. By contrast, instructing a customer to self-install a filter is simple and cheap, provided the service provider knows which customers lack such filtering. Therefore, a real need exists for a diagnostic that can detect the absence of a microfilter on a DSL line. Moreover, for practical reasons, the diagnostic technique should be automated and should be controlled from the CO.